The present invention relates to supported catalysts for the decomposition of nitrous oxide. These can be used, in particular, in industrial processes in which nitrous oxide is formed, for example the preparation of caprolactam, adipic acid or nitric acid.
In the industrial oxidation of ammonia, for example during the course of the production of nitric acid, formation of the desired nitrogen monoxide NO is accompanied by formation of the undesirable nitrous oxide N2O. This contributes in a not inconsiderable measure to the degradation of stratospheric ozone and to the greenhouse effect. Further sources of nitrous oxide are industrial oxidations using nitric acid as oxidant, for example as carried out in the preparation of adipic acid.
Although the relative proportion by volume of N2O in the climate-relevant trace gases in the earth's atmosphere is less than 0.1% by volume (CO2: 98.7% by volume, CH4: 1.2% by volume), its greenhouse potential is 310 times that of CO2 and the relative proportion of nitrous oxide therefore amounts to about 30% of the contribution of CO2 to the additional greenhouse effect caused by human beings.
Technical solutions for reducing nitrous oxide emissions, particularly in nitric acid production since this process is the largest source of industrial nitrous oxide emissions, are not only required for reasons of environmental protection but are now also demanded by legislators.
An example of a gas-phase reaction which is carried out on a large scale in industry and is associated with N2O problems is the preparation of nitric acid. This is generally carried out on the industrial scale by the Ostwald process by means of catalytic oxidation of ammonia over Pt/Rh catalysts. Here, NH3 is oxidized very selectively to NO which is then oxidized to NO2 during the course of the further process and the NO2 is finally reacted with water in an absorption tower to give nitric acid. The Pt/Rh catalysts are configured as fine gauzes and are stretched over a wide area in a burner. A gas mixture of typically about 8-12% by volume of ammonia in air is passed through the gauzes, with a temperature of about 850-950° C. being established at the gauzes as a result of the exothermic reaction.
An overview of the course of nitric acid production and its various process variants is given in Ullmanns Encyclopedia of Industrial Chemistry, Vol. A 17, VCH Weinheim (1991).
There are in principle three different processes and plant engineering possibilities for reducing the N2O emissions in the offgases from industrial plants such as plants for nitric acid production:
1. Primary Measure
Selective oxidation of ammonia to nitrogen monoxide and avoidance of the undesirable formation of nitrous oxide by modification of the chemical composition of the oxidation catalyst.
2. Secondary Measure
Reduction of the N2O content of the process gas by installation of a catalyst which selectively decomposes nitrous oxide into its constituents N2 and O2 between the noble metal gauzes which are usually used in the oxidation of ammonia and at which the oxidation takes place and the absorption tower, and also upstream of the first heat exchanger unit after the ammonia oxidation. The process temperature and, associated therewith, the required operating temperature of the catalyst is comparatively high here, for example in the range from 800 to 1000° C.
3. Tertiary Measure
Catalytic decomposition of the N2O present in the offgases leaving the absorption towers. This is an offgas purification arranged downstream of the actual production process. The offgas temperature and thus also the operating temperature of the catalyst is comparatively low here and varies, depending on the plant type, in the range, for example, from 200 to 700° C. In this offgas purification, the catalyst is arranged between absorption tower and stack, preferably between absorption tower and tailgas turbine and in particular just before the tailgas turbine.
While variant 1 can be achieved by variation of oxidation catalysts and/or by a change in the pressure and temperature conditions in the process, variants 2 and 3 require the use of specific catalysts for the selective decomposition of N2O, and these have to meet all requirements predetermined by the process.
In industrial oxidation processes using nitric acid as oxidant, for example in the preparation of adipic acid, large proportions of nitrous oxide which can amount to up to 50% by volume of the respective offgas are formed in the process. The temperatures of the offgas can increase to up to about 900° C. as a result of the exothermic decomposition of nitrous oxide. A suitable N2O decomposition catalyst therefore has to be active in this temperature range and has to be suitable for long-term use in this temperature range.
Intensive research on catalysts which make it possible to decompose N2O into the unproblematical components N2 and O2 has been carried out over the years. The range of possible catalyst materials extends from catalysts which contain noble metals and have preferably been applied to nonmetallic inorganic support materials through microporous framework silicates (zeolites) which have been cation-exchanged or contain metal oxides to transition metal oxides and mixed oxides having a perovskite or perovskite-like structure or a spinel structure.
The in-principle suitability of many of the catalyst materials mentioned has been demonstrated in the technical and patent literature but elevated pressure, very high operating temperatures and corrosive conditions place particularly great demands on the catalysts not only in respect of their catalytic activity and selectivity but in particular also in terms of their thermal and chemical stability over prolonged periods of time.
Catalysts for use according to the secondary measure (hereinafter referred to as “secondary catalyst”) in nitric acid plants first of all have to have a high thermal stability to be able to operate over the long term at the high temperatures of typically from 800 to 1000° C. which are required. This thermal stability is possessed neither by simple noble metal catalysts, which are deactivated or vaporize at these temperatures, nor by zeolite or hydrotalcite structures, whose framework structure is destroyed at these temperatures. High-temperature-resistant ceramic catalysts are therefore a possible alternative.
Secondary catalysts often comprise a high-temperature-resistant ceramic support material which can itself have catalytic properties but does not necessarily have to have such properties and also one or more active components. The catalytically active component can be distributed homogeneously in the ceramic matrix or be present as a layer applied to the surface. This results in a further requirement which a secondary catalyst has to meet, namely that no chemical reaction between ceramic support and active component, which would inevitably lead to deactivation, can take place at the high use temperatures.
It is known from the literature that especially transition metal oxides and in particular cobalt oxide CO3O4 are very good catalysts, i.e. active components, for the decomposition of N2O. Mixed oxides containing transition metals and having a perovskite structure, a perovskite-like structure or a spinel structure have also been described and examined in detail many times (N. Gunasekaran et al., Catal. Lett. (1995), 34 (3,4), pp. 373-382).
The disadvantage of the comparatively high price of these catalysts is countered in the prior art by the costly active components either being dispersed in an inexpensive ceramic matrix or applied to the surface of such a ceramic support material. However, in the majority of cases, the studies on these catalysts have been restricted to operating temperatures in the range from 300 to 600° C. At elevated temperatures as occur, for example, during use as secondary catalyst in plants for the preparation of nitric acid, is new problems arise, especially unsatisfactory sintering stability and the tendency for chemical reactions to occur between support material and active component, as a result of which the catalyst can lose its activity (deactivation).
In the specific case of the secondary catalyst (decomposition of N2O in the presence of NOx, the target product of the process), there is the further important requirement of selectivity of the N2O decomposition over the decomposition of NOx, which the catalyst has to meet.
A further requirement which the bed of secondary catalysts has to meet is a comparatively low weight of the bed, since only a limited space is available in the plant and the plant components can be stressed to only a limited extent by the weight of the catalyst. Low catalyst weights can in principle be achieved by the use of catalysts having a high activity and/or low bulk density.
The objective in the development of a secondary catalyst is thus to find a material system and a production process by means of which the challenges mentioned can be met. Here, the use of Co3O4 and/or Co-containing mixed oxides (e.g. perovskites of the general composition La1-xAxCo1-yByO3 where A=Sr, Ca, Ba, Cu, Ag; B=Fe, Mn, Cr, Cu; x=0 to 1 and y=0 to 0.95), which are excellent in respect of their catalytic activity for the decomposition of N2O, is particularly problematical since the irreversible chemical reaction of Co3O4 with many support materials, e.g. Al2O3, which occurs at temperatures above about 900° C. leads to a loss of catalytic activity.
WO-A-00/13,789 describes a secondary catalyst comprising alkaline earth metal compounds (preferably MgO) as support material. This has the advantage that MgO itself has some catalytic activity for the decomposition of N2O and the proportion of costly active components can therefore be reduced. A disadvantage is that the selectivity is not 100% and NOx is sometimes also decomposed. Furthermore, long-term tests on this material system under realistic conditions show that, here too, Co ions migrate from the active phase Co3O4 into the MgO lattice and an Mg1-xCoxO solid-state compound is formed, which is associated with deactivation of the catalyst.
A similar material system comprising cobalt oxide as active component and magnesium oxide as support material is described in U.S. Pat. No. 5,705,136. Here too, the problems of unsatisfactory sintering stability in the high-temperature range are recognized, so that the catalysts described in this document are suitable for use at temperatures in the range from 400 to 800° C. but not for high-temperature use in nitric acid plants.
Irreversible solid-state reactions between transition metal oxides and ZrO2 are also known, so that zirconium oxide (mentioned as support material in JP-A-48/089,185) is also ruled out in the high-temperature range.
WO-A-02/02,230 claims a catalyst comprising Co3-xMxO4 (M=Fe, Al and x=0 to 2) as active component on a CeO2 support. In actual fact, no reaction between active component and support material takes place here at a use temperature of 900° C. and the selectivity of the catalytic reaction is also improved by CeO2. However, price, availability and the tremendous weight are problematical for practical use of a catalyst comprising a solid support composed of CeO2.
A wider range of possible catalyst materials is available for the elimination of N2O from the tailgas of nitric acid plants because of the lower offgas temperatures and therefore operating temperatures. Furthermore, the requirement for selectivity over other nitrogen oxides no longer applies. However, there is instead the new problem of the deactivating influence of NO on the decomposition of N2O.
In Greenhouse Gas Control Technologies, Elsevier Science Ltd. 1999, pp. 343-348, F. Kapteijn et al. describe cobalt- and rhodium-doped hydrotalcite structures as active N2O catalysts at low operating temperatures. A further very detailed publication by F. Kapteijn's group may be found in Applied Catalysis B: Environmental 23 (1999), pp. 59-72. The disadvantage of hydrotalcites is explicitly stated in, for example, EP-A-1,262,224: gas constituents such as oxygen, water or NO adversely affect the N2O conversion over the catalyst. Use in real industrial offgases is therefore virtually ruled out.
Particular attention has hitherto also been paid to zeolites. Active species such as Fe, Cu or Co can be incorporated into these microporous framework silicates by cation exchange or mechanical mixing, which in the combination of active component/pore structure gives very active catalysts for the decomposition of N2O. Thus, for example, US-A-2003/0143142 describes an Fe-containing zeolite which is used as tertiary catalyst and displays no deactivation by NO but instead the decomposition of N2O over the catalysts is even promoted by the presence of small amounts of NOx. The disadvantage of zeolites is their sensitivity to water vapor present in the offgas and their limited thermal stability, which, taking account of the minimum temperature required for the decomposition of N2O, gives a limited temperature window in which these zeolite catalysts can be used.
Supported noble metal catalysts are likewise suitable as tertiary catalysts, but are many times as expensive as ceramic catalysts which are free of noble metals.
DE-A-100 06 103 describes a tertiary catalyst which is produced by mechanical mixing of MgO and cobalt oxide (preferably Co3O4) or of precursors of these oxides by means of dry pressing and subsequent heat treatment. At use temperatures in the range from 350 to 550° C., the problem of a solid-state reaction between the two oxides does not occur. However, these catalysts are found to be susceptible to NOx in the offgas. Although the deactivation in respect of the decomposition of N2O which occurs here is reversible, this reversal is difficult to carry out under industrial use conditions.
Studies on the catalytic decomposition of nitrous oxide into nitrogen and oxygen are known from Applied Catalysis B: Environmental, Elsevier, vol. 64, no. 3-4; pp. 234-242 (2006). The catalyst described in this document is produced by impregnation of a monolithic support which has been provided with a “washcoat” of gamma-aluminum oxide and cerium oxide and has subsequently been impregnated with an active transition metal component, for example a cobalt salt. On heating this catalyst, part of the active transition metal component is incorporated into the aluminum oxide layer.
Applied Catalysis A: General, Elsevier, vol. 301, no. 2; pp. 145-151 (2006) describes the use of catalysts supported on aluminum oxide for the oxidation of CO or of hydrocarbons.
FR 2 860 734 A describes the use of supported catalysts for the combustion of soot, in particular soot in the exhaust gases from motor vehicles.
EP-A-1,147,813 describes a shaped ceramic catalyst body for the selective decomposition of N2O. Here, supports having a high proportion of MgO are used.